Additional series which is calculated based on other series

[hindseb]

Checklist

• I am running the latest version of the card
• I checked if there isn't another feature request opened with the same request
• I checked that the feature is not already available in the dev branch

Is your feature request related to a problem? Please describe.
I am trying to show the efficiency of my heating system. Thus, I am showing the heating duration per unit (living-, bedroom & office) and the heating degree days.
To show the efficiency, I would like to calculate a ratio, ie the sum of all heating units, over the degree days. In the below example, it would be (8.5 + 9.4 + 4.4) / 57.5 = 0.3878

As I would like to show this information based on my grouping (day, week, month etc), I would like to calculate this in the graph and not as a new template.

Describe the solution you'd like
To be able to add series, which utilizes the data of existing series. Sample

• entity: [no need of an entity]
  type: column
  yaxis_id: first
  statistics:
  type: change
  period: week
  align: end
  data_generator: |
  (series[1].data + series[2].data+series[3].data) \ series[0].data

Thanks for considering this.

`type: custom:apexcharts-card
experimental:
brush: true
header:
show: true
title: ApexCharts-Card
show_states: true
colorize_states: true
graph_span: 90day
span:
end: day
stacked: true
yaxis:

• id: first
  decimals: 0
  min: 0
  max: 150
• id: second
  opposite: true
  min: 0
  decimals: 0
• id: third
  opposite: true
  min: 0
• id: fourth
  opposite: false
  min: 0
  decimals: 0
  max: 140
  series:
• entity: sensor.heating_degree_days
  type: line
  yaxis_id: second
  statistics:
  type: change
  period: week
  align: end
  stack_group: b
• entity: sensor.living_room_heating_hours_today_total_increasing
  type: column
  yaxis_id: first
  statistics:
  type: change
  period: week
  align: end
  show:
  datalabels: false
  stack_group: a
• entity: sensor.bedroom_heating_hours_today_total_increasing
  type: column
  yaxis_id: first
  statistics:
  type: change
  period: week
  align: end
  show:
  datalabels: false
  stack_group: a
• entity: sensor.office_heating_hours_today_total_increasing
  type: column
  yaxis_id: first
  statistics:
  type: change
  period: week
  align: end
  show:
  datalabels: false
  stack_group: a
  `